1. Field of the Invention
This invention pertains to material handling, and more particularly to apparatus for controlling the movement of discrete workpieces.
2. Description of the Prior Art
An inherent characteristic of the process of manufacturing many products is the transfer of partially completed products in the form of workpieces between different work stations. At each work station, the workpiece is subjected to one or more operations that ultimately result in a finished product. Accordingly, a vital part of product manufacturing is the movement of workpieces through manufacturing machinery.
The workpiece is typically transported along an elongated path having a length and configuration suitable for the necessary manufacturing operations. A common feature of workpiece transport systems is the intermittent manner in which they move the workpieces along the path through the manufacturing machinery. The workpiece transport system must not only move workpieces between the various work stations, it must also stop the workpieces at the work stations and maintain them in position there while the workpieces undergo the necessary manufacturing operations. Various stopping and locating mechanisms are often used to position and rigidly hold the workpiece in place at a work station. At the conclusion of the particular operation, the workpiece is released to be transported downstream to the next work station. Typical applications of workpiece transport systems include transfer lines used in conjunction with machine tools for machining workpieces. Other systems are employed to transport individual components between welding stations at which the components are joined together into a desired structure. A particularly important application of workpiece transport systems is the modern assembly machine.
Various types of transport systems have been developed to move workpieces along the paths of the different types of manufacturing machinery. In some transport systems, continuous chains are used to propel the workpiece along the required path. Typical chain-type equipment is produced by Gilman Assembly Automation of Janesville, Wis.; Robotrac, Inc. of Addison, Ill.; and by IT Equipment, Inc. of South Bend, Ind.
In other workpiece transporting equipment, the workpieces are supported on and propelled by rollers that define the workpiece path. Typical suppliers of roller equipment include High Technology Associates of Houston, Tex.; Sponmech, Limited of Stourbridge, England; Automated Manufacturing Systems, Inc. of Fenton, Mich.; Harry Major Machine & Tool Company of Fraser, Mich.; IT Equipment, Inc. of South Bend, Ind.; and Rockwell International of Farmington, Mich.
In all the foregoing systems, a main drive mechanism continuously urges the rollers or chains to propel the workpieces downstream. To stop a workpiece at a work station, suitable mechanical stops are employed. Upon striking a stop, the workpiece is halted. However, because the main drive mechanism continues to operate, sliding motion is created somewhere in the system between the stationary workpiece and the moving main drive mechanism. In chain type equipment, sliding occurs between the chains and the workpiece. In roller type equipment, the rollers are halted, as by means of one or more friction clutches, so that no sliding occurs between the workpiece and the rollers. When a particular workpiece is stopped, other workpieces upstream from it are also stopped, usually by other positive stops. Upon the release of a workpiece, the upstream workpieces are also released for downstream propulsion. A suitable controller monitors the locations of the workpieces and appropriately actuates the various mechanical stops to control workpiece flow through the system.
The prior workpiece transport systems generally work quite well, and they are in widespread use. Nevertheless, they do possess certain disadvantages. Monitoring the locations and movement of numerous workpieces and controlling the mechanical stops requires a large and sophisticated controller. The cost of the electrical wiring between the sensors and controller and of the fluid piping and related components associated with the stops is very high. In addition, manufacturing, assembling, and servicing the numerous mechanical stops is undesirably expensive.
Roller type transport systems require the workpiece paths to be divided into relatively short sections, with each section having its own drive motor. If a component in any drive fails, the entire transport system is put out of service until the failed drive component is repaired or replaced. In many applications, sliding between the chains or rollers and the workpiece is highly undesirable. The friction clutches associated with some prior transport systems have the disadvantage of requiring periodic adjustment. Even with care the clutches are prone to transmitting variable torque due to contamination and misadjustment.
The Buschman Company of Cincinnati, Ohio, manufactures a conveyor that utilizes air controlled clutches and rollers, and the conveyor is divided into zones. A single sensor in each zone controls when the rollers of the adjacent upstream zone are to rotate or to remain stationary, and thus whether a workpiece is to be propelled along the adjacent upstream zone. However, pneumatic control is not well accepted for assembly machine applications, and little bottom access is available on such installations. Further, the capacity for transporting workpieces and pallets is undersirably low.
Thus, a need exists for improvements in workpiece transport equipment.